The present invention relates to an electron microscope and a method of controlling the same.
In general, a monochromator includes an energy filter serving as a spectroscopic part for an electron beam and an energy selection slit. The electron beam incident on the monochromator is split by the energy filter to form a spectrum corresponding to the energy distribution of the electron beam on the plane of the energy selection slit placed on an energy dispersion surface. In the energy selection slit, only an electron beam having a specified energy width corresponding to the slit width of the energy selection slit passes through the slit. This monochromatizes the electron beam (see, e.g., JP-A-2016-39118).
FIG. 6 is a diagram schematically illustrating an example of a configuration of a conventional monochromator 1100 with an internal electron gun. Note that, in FIG. 6, the illustration of an electron beam EB downstream of an energy selection slit 1104 is omitted.
The monochromator 1100 is embedded between an electron source 1002 and an acceleration tube (not shown) that accelerates the electron beam EB. On the incident side of the monochromator 1100, an incident-side electrode 1006 is provided. In addition, between the electron source 1002 and the incident-side electrode 1006, an extraction electrode 1004 is provided.
By applying a voltage to the extraction electrode 1004, an intense electric field is generated in the tip portion of the electron source 1002. Due to the electric field, the electron beam EB is emitted from the tip portion of the electron source 1002 (tunnel effect). The electron beam EB emitted from the electron source 1002 is caused by an electrostatic lens 1008 generated between the incident-side electrode 1006 and the monochromator 1100 to have parallel course and be incident on the monochromator 1100.
The monochromator 1100 has an energy filter 1102, an energy selection slit 1104, and a casing 1106 surrounding the energy filter 1102 and the energy selection slit 1104.
The energy filter 1102 generates a deflection field in the optical path of the electron beam EB. The energy filter 1102 splits the electron beam EB using the different pathways thereof resulting from the different speeds of electrons in the deflection field and projects, on the energy selection slit 1104, a spectrum corresponding to the energy distribution of the electron beam EB emitted from the electron source 1002. The resolution of the energy filter 1102 is about 10 μm/eV. As a result of passing through the slit of the energy selection slit 1104 having a width of several micrometers to submicrometers, the electron beam EB has an energy distribution corresponding to the width of the slit. As a result, the electron beam EB is monochromatized.
In an electron gun, by changing the voltage applied to the extraction electrode 1004, the electric field generated in the tip portion of the electron source 1002 is changed to allow a beam current (emission current) emitted from the electron source 1002 to be controlled.
The electric field generated in the tip portion of the electron source 1002 depends on the voltage applied to the extraction electrode 1004 and the radius of curvature of the tip portion of the electron source 1002. Accordingly, when the shape of the tip portion of the electron source 1002 has changed with time, the beam current emitted from the electron source 1002 is also changed.
To prevent this, in the electron gun, the voltage applied to the extraction electrode 1004 is controlled such that the beam current has a specific value. Specifically, the voltage applied to the extraction electrode 1004 is changed based on a change in the shape of the tip portion of the electron source 1002, i.e., based on a change in the beam current emitted from the electron source 1002.
When the monochromator 1100 is mounted in the electron gun, a change in the voltage applied to the extraction electrode 1004 affects the intensity of an electric field on the incident side of the monochromator 1100. In other words, the change in the voltage applied to the extraction electrode 1004 affects the electron optical system of the electron gun.
For example, when the voltage applied to the extraction electrode 1004 is changed to change the intensity of the electric field on the incident side of the monochromator 1100, the effect of an electrostatic lens formed between the extraction electrode 1004 and the incident-side electrode 1006 is changed. As a result, the convergence plane of the electron beam EB which should intrinsically coincide with the plane in which the energy selection slit 1104 is provided is displaced therefrom (see FIGS. 7 and 8). Consequently, the electron optical system of the monochromator 1100 falls out of optimal conditions (see FIG. 6) to degrade the performance of the monochromator 1100.
For example, as a result of a change in the voltage applied to the extraction electrode 1004 and a consequent change in the intensity of the electric field on the incident side of the monochromator 1100, the electron beam EB may be affected by axial misalignment resulting from an error in mechanically assembling the electron source 1002, the extraction electrode 1004, and the incident-side electrode 1006. As a result of the change in the voltage applied to the extraction electrode 1004, the electron beam EB is deflected under the influence of axial misalignment between the electron source 1002 and the extraction electrode 1004 and axial misalignment between the extraction electrode 1004 and the incident-side electrode 1006. Consequently, the deflected electron beam EB is incident directly on the energy filter 1102 (see FIG. 9).
As a result of a change in the voltage applied to the extraction electrode 1004 and a consequent change in the intensity of the electric field on the incident side of the monochromator 1100, an amount of deflection of the electron beam EB due to the axial misalignments described above is also changed to also change the angle of incidence of the electron beam EB with respect to the energy filter 1102. As a result, the electron optical system of the monochromator 1100 falls out of the optimal conditions (see FIG. 6) to degrade the performance of the monochromator 1100.